Add dynamic config setting to force selected API mode for auto-forwarding

[robholland]

What changed?

Added a dynamic config variable to force selected API mode for auto-forwarding.

Why?

This helps in use cases where poll requests should not be forwarded when the cluster is not active for this namespace.

How did you test it?

• built
• run locally and tested manually
• covered by existing tests
• added new unit test(s)
• added new functional test(s)

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Note

Introduces a per-namespace dynamic config to constrain auto-forwarding to a whitelist of APIs, even when the cluster policy is set to forward all APIs.

• Adds system.forceNamespaceSelectedAPIAutoForwarding and plumbs it through frontend config and interceptor.NewRedirection
• Extends DC redirection policies (dc_redirection_policy.go) to accept selectedForNS; when all-apis-forwarding is enabled, selectedForNS forces whitelist-only forwarding for that namespace
• Updates RedirectionPolicyGenerator, policy constructors, and interceptor wiring to pass the new selector
• Adds/updates unit tests validating override behavior and wiring (dc_redirection_policy_test.go, redirection_test.go, nexus_handler_test.go)

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